1. Field of the Invention
The present invention relates to an electrophotographic developer and an image-forming method using the above developer, specifically to an electrophotographic developer which can provide a good image when using an electrostatically charged-image holder (photoreceptor drum) having a small diameter and an image-forming method.
2. Description of the Related Art
In general, in an electrophotography, a toner having a prescribed charging property is developed with a developing unit in order to visualize a latent image formed on a photoreceptor. A toner image is fixed on paper by heating and pressing in a fixing system, whereby a copied image is obtained. In particular, large factors affecting an image quality and a grade of a copied image include a developing step for accurately developing an electrostatic latent image on an electrostatically-charged image holder, a transferring step for transferring the developed developer on a transferred material by means of various transferring devices and a fixing step for fixing the developer on the transferred material. Among them, the developing step is an important step as a first step for visualizing a latent image.
Developers used in a developing step are single component or two component developers, and toners are used for these developers. In recent years, various developing methods have been proposed while advanced are a raise in an image quality, a reduction in a cost and an elongation in a life. A raise in an image quality by a decrease in a particle diameter is proposed in, for example, Registered Japanese Patent Publications No. 2759490, 2759516 and 2763318. Proposed in these patented inventions are techniques to achieve a raise in an image quality by a reduction in a particle diameter of a toner, an inhibition in contamination caused by scattering of toners and maintaining of an image quality and a decrease in contamination in the machine even in use over a long period of time.
A raise in an image quality by a reduction in a particle diameter of a toner which is proposed above is considered to be an effective means for visualizing an electrostatic latent image which is constituted from dots by digitization in recent years. However, a reduction in a particle diameter of a toner makes a production step thereof complicated and therefore leads to production at a high cost. In a crushing method which has so far been used well, a kneaded matter of various toner components is roughly crushed to 1 to 5 mm by a crusher and further finely crushed through a step in which means such as a mechanical system and a collision system are combined, and then it passes through a classifying step such as a flow system, whereby a desired toner particle size is obtained. Usually, it is possible to readily obtain a toner having a volume average particle diameter of 8 to 12 μm by the crushing method described above, but further measures are required in order to obtain a toner particle size of 12 μm or less by the above crushing method, and the facilities become complicated. This turns the above method into a method having an inferior productivity and a high cost. Further, a polymerization method is used as a method for obtaining a toner having a small particle diameter, but various organic solvents are used in many cases, and waste water treating facilities therefor, complicated maintenance thereof and the inferior yield turn the above method into high cost production in terms of facilities therefor.
Further, it is very difficult to handle a toner having a small particle diameter because of specific powder characteristics thereof. Brought about are a deterioration in the image caused by the inferior fluidity, particularly a degradation in the fluidity at a high temperature and a high humidity and a reduction in the image density caused by a rise in the charge amount at low temperature and a low humidity. Further, continuing copy-printing in many sheets expedites a rise in the charge amount and causes fusion or adhesion of a toner on a carrier surface. This markedly brings about an inhibition in charging, an increase in contamination in the machine caused by scattering of a toner, a rise in the background density and a deterioration in the image caused by a reduction in the image density and shortens a life of a developer. As described above, a decrease in a particle diameter of a toner makes it possible to raise the image quality for a while but leads to a toner having a high cost and a short developing life. Further, restriction and the construction of a new mechanism are required to the process conditions and the image-forming system themselves, which in turn can not help resulting in elevating a cost required for one sheet of a print.
In recent years, space saving and a reduction in a size of a printing system in a fixing step of a developer on a transferring material are promoted. This allows all parts and units to be inevitably reduced in size. In particular, a reduction in a size in a process (charging, exposing, developing, transferring and removing charge) is important. Among them, more rapid and surer developing is required by reducing a size of the above holder by reducing a radius of curvature in an electrostatically charged-image holder to narrow a mass part (development effective range) where the electrostatically charged-image holder is disposed oppositely to the developer carrier.
Further, an electrostatically charged-image holder and a developer carrier rotate in a developing region in directions reverse to each other in a certain case to carry out development. In this case, troubles are more notably caused if a toner has a short developing time and an inferior developability, and the probability of the troubles results in growing large.
Narrowing of a development effective range strengthens the tendency that a toner having a low charge amount in a developer is selectively developed, so that the toner having a high charge amount remains selectively in the developer or a developing unit. It in turn results in allowing the toner having a relatively large particle diameter to be selectively developed and the toner having a small particle diameter to remain in the developer. That is, remaining of the toner having a small particle diameter contaminates the surface of a toner carrier and brings about an inhibition in charging of the toner in the fresh developer fed, and it in turn causes various kinds of the problems described above.
Usually, the larger the radius of curvature in an electrostatically charged-image holder or a developer carrier and the longer than a distance between the electrostatically charged-image holder and the developer carrier the head distance of the developer, the more the range (development effective range) where the developer comes in contact with the electrostatically charged-image holder can be broadened and the larger the chance in which the developer transfers into the electrostatically charged-image holder and is developed results in growing. Further, the larger the peripheral speed ratio of the developer carrier to the electrostatically charged-image holder, the larger the chance in which the developer transfers into the electrostatically charged-image holder and is developed results in growing as well. However, the more the chance in which the developer is developed is increased, the larger the chance in which the toner transfers from the electrostatically charged-image holder into the developer carrier results in growing.
Accordingly, in order to maintain a developing state well, the electrostatically charged-image holder described above has to be balanced with the developer carrier in terms of a transferring state in a two-way direction.